The present invention relates generally to devices and driving methods for miniature motors and in particular to electromechanical motors based on repetitions of small steps.
There are numerous applications with the need for extremely miniaturised motors that are able to make controlled fine positioning. Among these, portable consumer devices, such as e.g. cameras, have additional demands for low power consumption, low weight and price. The required motion is typically linear but often a rotating motor combined with a motion-converting mechanism such as a lead-screw is used. The motion range is often in the order of millimeters. There is yet no real miniature motors presented which fulfils all demands above.
Generally, the use of a solid actuator material and some type of magnification mechanism appear to be the best solution for the desired miniature motors. A solid actuator such as a PZT element has a very high energy density and the size of a motor could therefore be minimised. Typically, existing actuator materials with high energy density are not able to change its shape more than a tenth of a percent, which makes it difficult to make an optimised construction with small external dimensions. One component that has been used in numerous applications is a piezoelectric bimorph element since a high internal motion magnification can be achieved in the bending mode. In U.S. Pat. No. 4,291,958, a bimorph cantilever in combination with a magnifying lever is suggested for the focusing of cameras. However, the necessary stroke of such a focusing device results in a poor stiffness of the device. In U.S. Pat. No. 4,339,682, a motor based on two bimorphs connected by an elastic member to drive a rotor has been presented. Stepwise movements magnify the motion. Apart from the space demands of this construction, the conversion of a rotating motion into a linear motion does normally result in a reduced performance. Bimorphs operating in the bending mode in combination with teeth on both rotor and drive elements or only on the rotor have been used to construct mechanical stepping motors, as disclosed e.g. in the abstracts of JP 61-177178 and JP 2-142365. A method to improve the energy transfer from a bending bimorph is suggested in EP 0993055. This improved bimorph is intended to be used in an ultrasonic motor. A motor intended for applications, which demand extremely small sizes, was constructed and presented in SE9300305-1. Rotation or linear translation is performed by stepwise motion with bimorph elements in direct contact with the object to be moved. In this invention, the bimorph elements are driven in such a way that the contact point of the bimorph element moves in two dimensions, i.e. the bimorph is used both in the bending and the longitudinal direction.
In several applications, space is a crucial factor, and there is in many cases not enough space to supply enough rigid supports for two-dimensional cantilever bimorphs according to prior art. There is thus a need for simple drive elements that can operate in narrow spaces with limited mechanical support.
There are basically two properties of the support to consider. Firstly the bending deflection of the bimorph tip due to the flexibility in the support. Secondly the bending stiffness of the support in relation to the equivalent mass with respect to support bending. If the flexibility of the support would allow the bimorph tip to move as much as what is achieved by piezoelectric activation of the bimorph, then there are only a few ways to create stepwise motion. Either a stick-slip mechanism could be used or an inertial mechanism where the support bending equivalent mass is utilised, which means operation at frequencies higher than the resonance frequency of the support. The resonance frequency of the support will typically be close to the resonance frequency of the bimorph itself since the spring constants and masses have to be about the same. In practice the design will be extremely critical and at miniature size the performance rather poor. If a stiffer support is used, the resonance frequency of the support will further increase and the available operating frequency range will decrease or even disappear. There is essentially just one solution that gives freedom in design and allows for performance optimisation and that is a very high bending stiffness of the support in relation to the stiffness of the bimorph itself. To get this desired stiffness the support will be rather large or complex.
An object of the present invention is to provide electromechanical elements, e.g. piezoelectric elements, which are able to operate satisfactorily with limited mechanical support and which are operable in limited radial spaces. A further object is to provide electromechanical elements with an improved efficiency and a higher ratio between force and volume. Another object of the present invention is to provide electromechanical elements having more flexible driving modes.
The above objects are achieved by devices according to the enclosed patent claims. In general words, electromechanical, preferably piezoelectric, elements are used, which comprises at least two movable parts or displacement portions interconnected by a passive part. Each displacement section comprises at least one bimorph, where the active volumes extend in parallel out from the passive part, and so are the electrodes arranged between the electromechanical layers. The displacement portions are positioned in substantially the same plane and parallel to the surface to be moved. Contact portions are arranged at the central passive part and at the displacement portions. In one embodiment, the central contact portion is an actuating surface and the other are attaching portions for attachment to a stator. In another embodiment, the central contact portion is an attaching portion and the other are actuating surfaces. The actuating surfaces are in both cases movable relative to the attaching portions in two dimensions.
Advantages with the present invention are that the motor can be manufactured very small, and with simple attachment solutions. The elements can easily be operated with dynamic as well as non-dynamic drive mechanisms, and a very high efficiency is possible to reach. Multi-axial motion can be made by the generic types of elements with which it is possible to either increase performance or reduce the number of voltage signals.